This invention pertains to the processing of certain metal alloys to produce an orange peel-like textured surface. More specifically, this invention pertains to the controlled cold working of a superplastic-formable (SPF) metal alloy sheet so as to yield a roughened textured surface in at least a portion of the sheet upon stretch forming at a suitable elevated temperature and strain rate.
There are families of metal alloy compositions that when subjected to suitable thermomechanical processing display extraordinary elongation or plastic deformation properties. They are then said to have superplastic forming properties (or to be superplastically formable, either phrase sometimes abbreviated as SPF). Some aluminum, iron, magnesium and titanium compositions have such properties. Often SPF materials have a metallurgical microstructure characterized by a matrix phase of the major constituent such as aluminum, or of a solid solution of the major phase and minor alloying elements, and very finely divided dispersed phase of intermetallic material. Materials with such a microstructure are sometimes called pseudo-single phase materials because of the very small dispersed phase. In sheet form, such materials can be cold rolled to reduce thickness and increase length while breaking up the existing grains and storing the work energy in the microstructure of the sheet. Then, upon heating to a suitable temperature, the strain is relieved by recrystallization to yield a very fine grain microstructure susceptible to forming operations at a suitable temperature to produce complex shapes from the sheet in which portions have experienced extraordinary elongation and deformation.
Certain SPF titanium alloy sheet compositions (e.g., Ti-6% Al-4% V alloys) have probably been the first materials to be used commercially. They have been used in the aerospace industry because of their very favorable strength-to-weight ratio. These sheet materials are formed at suitable elevated temperatures in the range of, for example, 800xc2x0 C. to 900xc2x0 C. into complicated one-piece shapes that often eliminate the previous need to form several smaller pieces and join them together. The sheets experience strain rates of 10xe2x88x924 to 10xe2x88x923 and elongation of several hundred percent. The need of the aerospace industry for strong lightweight parts has permitted the use of expensive alloys and relatively slow manufacturing processes. At present, however, SPF practices with titanium alloys have been too expensive for the lower cost requirements of the automobile industry.
Work has begun to adapt some aluminum alloys to lower-cost SPF processes and part manufacture. For example, AA 5083 has been formed by hot rolling of a cast ingot to a strip and subsequent severe cold rolling of the strip to a sheet material that is a precursor for SPF part manufacture. AA5083 have typical compositions, in weight, of about 4% to 5% magnesium, 0.4% to 1% manganese, 0.05% to 0.25% chromium, about 0.1% copper, and the balance aluminum. The cold-rolled sheets are heated to a suitable temperature of, e.g., about 500xc2x0 C. where recrystallization to a fine grain (about 10 m) microstructure quickly occurs and the sheet is warm enough to be formed with relatively high elongation for such alloys. The heated sheet is placed adjacent a suitable forming tool, secured at the edges and stretched against and into compliance with the forming tool using the pressure of a gas such as air, nitrogen or argon.
SPF practices for the stretch forming of aluminum alloys such as AA5083 are illustrated in patents such as U.S. Pat. No. 5,819,572 Krajewski, xe2x80x9cLubrication System for Hot Forming;xe2x80x9d U.S. Pat. No. 5,974,847 Saunders et al, xe2x80x9cSuperplastic Forming Process;xe2x80x9d and U.S. Pat. No. 6,047,583 Schroth, xe2x80x9cSeal Bead for Superplastic Forming of Aluminum Sheet,xe2x80x9d each assigned to the assignee of this invention. As suggested in these patents, the goal in developing SPF practices for the manufacture of aluminum sheet products on a commercial scale has been to make tear-free articles with unmarred surfaces. But it would also be useful to form sheet metal products with SPF-like capabilities that have textured, or uniformly roughened surfaces, in at least a portion of the article. In other words, there are applications for SPF-type formed parts that have an orange peel-like surface of decorative or anti-skid properties or the like.
This invention provides a method of forming a SPF-type metal alloy sheet of specified thickness so that at least a portion of the resulting product has a surface with a visible uniform rough texture like the skin of an orange. The invention is applicable to metal alloys that can be cold worked to a sheet stock precursor having a suitable strained microstructure that will recrystallize to a fine-grained microstructure with high elongation characteristics upon heating to a recrystallization (and forming) temperature. The practice of the invention involves predetermining the amount of cold work that is to be applied to the precursor sheet stock so that, upon heating to a superplastic-forming temperature for the material and subsequent stretch forming, the deformation of the sheet results in a desired shape and the textured surface. For applications such as the manufacture of automobile body panels, SPF aluminum alloys are preferred because of their combination of low weight, high strength and low cost.
The preparation of a superplastic-formable, aluminum alloy sheet stock usually begins with a casting of a suitable composition such as AA5083. The cast material is then reduced in thickness by hot rolling to a strip that may, for example, have a thickness in the range of 20 to 40 millimeters depending somewhat on the goal for the final thickness of the sheet. The hot rolled strip is then cold rolled, usually in stages with possible interposed anneals, to a final thickness in the range of about one to three or four millimeters. The result of overall thermomechanical processing is typically a coil of smooth surface aluminum sheet stock, the microstructure of which has been severely strained. This material is then ready to be heated to 500xc2x0 C. or so for stretch forming as described above. The effect of the heating is to promote recrystallization of the severely worked microstructure to a very fine grained material susceptible to appreciable elongation during deformation by stretching against the forming tool.
In conventional superplastic forming of aluminum sheet, the goal is to obtain a sheet stock of desired thickness that ends up with a suitably fine grained microstructure to sustain deformation and elongation at the various critical spots on the sheet to form the desired part with at least one smooth surface and without body tears, ruptures, undue thinning and the like. In order to assure suitable elongation for any SPF job, the current sheet stock rolling practice is to maximize the cold rolling strain imposed on the sheet stock consistent with the specified thickness of the sheet stock. Depending upon the shape of the part to be formed, the sheet stock may then have sufficient or excess elongation (i.e., formability) for the task.
In accordance with this invention, a sheet product is formed from a stock material having marginally less formability than SPF starting material. The shape of the product will not require the extensive deformation obtained in an SPF process, but the product, upon heating to a suitable elevated temperature and stretch forming, will have a generally uniformly roughened surface portion. The amount of cold rolling strain imposed on the sheet is carefully determined to be less than that required for optimum SPF deformation but sufficient to make the part and to yield the textured surface. In other words, the creation of the textured surface is the result of a cold working and thermal recrystallization strategy that produces a defective part as far as SPF processing is concerned, i.e., a part with a roughened surface. Manufactured sheet metal parts with rough surfaces have utility for decorative purposes, low slip applications, coating adhesion, controlled heat transfer and the like.
Other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments which follows.